A 28-year-old multigravida at 34 weeks of gestation presents with recurrent episodes of painless vaginal bleeding. Transabdominal ultrasound shows the placenta partially covering the internal cervical os. The patient is hemodynamically stable and the fetus is in cephalic presentation. Which investigation is most appropriate to assess placental invasion and guide management?

Explanation

## Investigation for Placenta Previa with Recurrent Bleeding at 34 Weeks ### Clinical Context **Key Point:** A multigravida at 34 weeks with partial placenta previa and recurrent painless vaginal bleeding requires accurate localization of the placenta relative to the internal os and assessment of placental vascularity. The patient is hemodynamically stable, allowing time for definitive imaging. ### Why Transvaginal Ultrasound with Color Doppler? **High-Yield:** Transvaginal ultrasound (TVS) with color Doppler is the **investigation of choice** for: - Precise measurement of placental edge-to-os distance (superior resolution vs. transabdominal US) - Identifying features of placenta accreta spectrum (PAS): loss of retroplacental clear space, lacunae, abnormal vascularity at the bladder-uterine interface - Guiding management decisions (expectant vs. delivery, mode of delivery) - Safe to perform in placenta previa when done carefully with a transvaginal probe — the probe does not reach the os and does not trigger hemorrhage (RCOG Green-top Guideline 27; ACOG Practice Bulletin 279) **Clinical Pearl:** The widely held belief that TVS is "contraindicated" in placenta previa is a myth. Multiple studies and guidelines confirm TVS is **safer and more accurate** than transabdominal US for evaluating the lower uterine segment and internal os in placenta previa. The probe is placed in the vaginal fornix, well away from the os. ### Comparison of Imaging Modalities | Modality | Role | Accuracy | Notes | |----------|------|----------|-------| | **Transabdominal US** | Initial screening | Moderate (~50–60%) | Bladder filling, maternal habitus limit accuracy | | **Transvaginal US + Doppler** | **Definitive assessment** | High (>90%) | Safe; best for os distance + invasion signs | | **Doppler US alone** | Adjunct | Moderate (~70%) | Less specific than TVS with Doppler | | **MRI** | Problem-solving adjunct | High for invasion | Used when TVS equivocal; not first-line | ### Why Not the Other Options? **Option A — Doppler ultrasound with uteroplacental vascularity:** Doppler assessment alone without TVS provides less precise localization of the placental edge relative to the os. TVS with color Doppler combines both anatomical and vascular information in a single examination. **Option B — Repeat transabdominal ultrasound in 2 weeks:** Serial transabdominal US is appropriate for asymptomatic previa surveillance but is inadequate for a patient with recurrent bleeding at 34 weeks who needs definitive assessment now. **Option D — Cine-MRI of the pelvis:** MRI is a valuable adjunct when TVS findings are equivocal or when posterior placenta limits ultrasound assessment. However, it is **not the first-line investigation** — TVS with color Doppler should be performed first. The stem does not describe prior cesarean section, abnormal Doppler, or equivocal ultrasound findings that would justify bypassing TVS and proceeding directly to MRI. **Clinical Pearl:** Per RCOG Green-top Guideline 27 and ACOG Practice Bulletin 279, TVS with color Doppler is the recommended investigation for confirming placenta previa and assessing for placenta accreta spectrum features. MRI is reserved as an adjunct when ultrasound is inconclusive. [cite: RCOG Green-top Guideline No. 27 (2018); ACOG Practice Bulletin No. 279 (2022); Williams Obstetrics 26e Ch 41]